Power down system and method for integrated circuits

ABSTRACT

A power down system and method for an integrated circuit that enables a power down mode to be maintained for a predetermined time is described herein. The power down system comprises an oscillator, a low power oscillator and an oscillator control circuit controlling both the oscillator and the low power oscillator. The oscillator control circuit including at least one real time counter. The oscillator control circuit being so configured that the oscillator is energized when said oscillator control circuit is in a normal mode and that, when a power down signal is received: a) the oscillator control circuit measures an oscillation frequency of the low power oscillator, b) the oscillator control circuit uses the measured oscillation frequency of the low power oscillator to set the real time counter so as to maintain the power down mode for the predetermined time, c) the oscillator control circuit turns off the oscillator and uses the low power oscillator for the duration of the power down.

FIELD OF THE INVENTION

The present invention relates to Integrated Circuits (ICs). Morespecifically, the present invention is concerned with a power downsystem and method for ICs.

BACKGROUND OF THE INVENTION

Many ICs have a power down mode that is used to save power and, in thecase of battery powered environments, extend the battery life of thesystem.

The transition from the power down mode to the normal mode is usuallycontrolled by an external interrupt signal or is pre-programmed at fixedintervals. The latter case is most common in sand alone systems where anIC must “wake up” at predetermined intervals to perform functions suchas monitor sensors or communicate with other devices, for example.

When an IC is required to wake-up at known intervals, some circuitry isrequired to be operating even during the power down mode to keep trackof time and to wake the IC up. This circuitry usually includes anoscillator that is sufficiently precise and a real time counter. Theoscillator is often a crystal oscillator since it is very precise and isnot sensible to environmental variations such as temperature and inputvoltage variations. A drawback of using a crystal oscillator is that itrequires a relatively large biais current (in the order of tens of μA)during the power down mode.

To overcome this drawback, Chapman et al. in their U.S. Pat. No.5,845,204 issued on Dec. 1, 1998 and entitled “Method and apparatus forcontrolling the wakeup logic of a radio receiver in sleep mode”suggested the use of two oscillators. A crystal oscillator for itsprecision and a low power ring oscillator that runs in the sleep mode.During the wakeup time, the low power ring oscillator is calibrated tocompensate for its inaccuracy. While Chapman's method and apparatus isan improvement over apparatuses using crystal oscillator during thesleep mode, it still has drawbacks related to the compensation of thering oscillator frequency.

OBJECTS OF THE INVENTION

An object of the present invention is therefore to provide a power downsystem and method for Integrated Circuits.

SUMMARY OF THE INVENTION

More specifically, in accordance with the present invention, there isprovided a power down system for an integrated circuit that enables apower down mode to be maintained for a predetermined time; said powerdown system comprising:

an oscillator;

a low power oscillator;

at least one environmental sensor; and

an oscillator control circuit controlling both said oscillator and saidlow power oscillator and receiving data from said at least oneenvironmental sensor; said oscillator control circuit including a realtime counter; said oscillator control circuit being so configured thatsaid oscillator is energized when said oscillator control circuit is ina normal mode; said oscillator control circuit being so configured that:

when a reset signal is received:

-   -   environmental data is acquired by said oscillator control        circuit and stored in a first memory; and    -   said oscillator control circuit measures an oscillation        frequency of said low power oscillator and stores the frequency        in a second memory;

when a power down signal is received:

-   -   environmental data is acquired by said oscillator control        circuit and compared with the stored environmental data;    -   should the acquired environmental data differ from the stored        environmental data by at least a predetermined tolerance, said        oscillator control circuit measures an oscillation frequency of        said low power oscillator and overwrites the data contained in        the second memory;    -   said oscillator control circuit uses the oscillation frequency        of said low power oscillator stored in the second memory to set        said real time counter so as to maintain the power down mode for        the predetermined time, and    -   said oscillator control circuit turns off said oscillator and        uses said low power oscillator for the duration of the power        down.

According to another aspect of the present invention, there is provideda power down method for an integrated circuit that enables a power downmode to be maintained for a predetermined duration; said integratedcircuit including an oscillator, a low power oscillator, at least oneenvironmental sensor, standard circuitry and an oscillator controlcircuit having a real time counter, memory and controlling both theoscillator and the low power oscillator and connected to the at leastone environmental sensor; said power down method comprising:

measuring initial environmental data;

measuring an initial oscillation frequency of the low power oscillatorand storing it in memory;

receiving a power down signal;

measuring environmental data and comparing with the initialenvironmental data;

should the environmental data differ from the initial environmental databy at least a predetermined tolerance, measuring an oscillationfrequency of the low power oscillator and overwrite the initialoscillation frequency in memory;

setting the real time counter as a function of the oscillation frequencyin memory so as to maintain the power down mode for the predeterminedduration; and

turning off the oscillation of the oscillator and the standard circuitryfor the duration of the power down mode, as determined by the real timecounter.

According to another aspect of the present invention, there is provideda power down system for an integrated circuit that enables a power downmode to be maintained for a predetermined time; said integrated circuitincluding standard circuitry; said power down system comprising:

an oscillator;

a low power oscillator; and

an oscillator control circuit controlling both said oscillator and saidlow power oscillator; said oscillator control circuit including a realtime counter; said oscillator control circuit being so configured that:

said oscillator is energized when said oscillator control circuit is ina normal mode;

when a measurement of an oscillation frequency of said low poweroscillator is required, the standard circuitry of the integrated circuitis turned off;

when a power down signal is received: a) said oscillator control circuituses the measured oscillation frequency of said low power oscillator toset said real time counter so as to maintain the power down mode for thepredetermined time, and b) said oscillator control circuit turns offsaid oscillator and uses said low power oscillator for the duration ofthe power down.

According to yet another aspect of the present invention, there isprovided a power down method for an integrated circuit that enables apower down mode to be maintained for a predetermined duration; saidintegrated circuit including standard circuitry, an oscillator, a lowpower oscillator and an oscillator control circuit having a real timecounter and controlling both the oscillator and the low poweroscillator; said power down method comprising:

receiving a power down signal;

when predetermined conditions are met, shutting down the standardcircuitry; measuring an oscillation frequency of the low poweroscillator; powering up the standard circuitry;

setting the real time counter as a function of the measured oscillationfrequency of the low power oscillator so as to maintain the power downmode for the predetermined duration; and

turning off the oscillation of the oscillator and shutting down thestandard circuitry for the duration of the power down mode, asdetermined by the real time counter.

Other objects, advantages and features of the present invention willbecome more apparent upon reading of the following non-restrictivedescription of preferred embodiments thereof, given by way of exampleonly with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended drawings:

FIG. 1 is a schematic bloc diagram of an IC provided with a power downsystem according to an embodiment of the present invention;

FIG. 2 is a flowchart illustrating the operation of the IC of FIG. 1;and

FIG. 3 is a timing diagram illustrating the total power consumption andthe crystal oscillator oscillation with respect to the different modesof operation.

DETAILED DESCRIPTION

Generally, the present invention proposes a power down system and methodfor ICs that uses two different oscillators. A high precisionoscillator, for example a crystal oscillator, is used when the IC is inits normal mode and a low power oscillator, for example a relaxation orRC oscillator, is used when the IC is in a power down mode. Since lowpower oscillators are usually less precise, harder to control and moresensitive to environmental changes, their frequency is measured and realtime counters are adjusted so that the predetermined interval betweenwake-ups is maintained. The low power oscillator frequency measurementmay be done at a predetermined duty cycle or can be triggered byenvironmental changes, as will be described hereinbelow.

Turning now to FIG. 1 of the appended drawings, an IC 10 including apower down system according to an embodiment of the present inventionwill be described.

The IC 10 includes standard circuitry 12 to enable the IC 10 to performits intended functions, a high precision oscillator in the form of acrystal oscillator 14, a low power oscillator in the form of a low powerrelaxation oscillator 16; an oscillator control circuit 18 connected tothe standard circuitry 12 and to both oscillators 14 and 16, andenvironmental sensors 22. The oscillator control circuit includes realtime counters 20, the purpose of which will be described hereinbelow.

The interconnection between the standard circuitry 12 and the oscillatorcontrol circuit 18 enables the standard circuitry 12 to send a powerdown signal to the oscillator control circuit 18 when a power down modeis requested. Furthermore, as will be understood by one skilled in theart, the oscillator control circuit 18 supplies to the standardcircuitry 12 at least one clock signal derived from the oscillators 14and 16 and other signals intended to “wake-up” the standard circuitry 12after a power down mode, as will be described hereinbelow.

The low power relaxation oscillator 16 may be, for example a Schmitttrigger with programmable frequency, limited power dissipation and powerdown control. Of course, other low power oscillators such as ringoscillators and RC oscillators could be used.

The environmental sensors 22 include, for example, temperature, voltageand IC process variation sensors. They are connected to the oscillatorcontrol circuit 18 to supply environmental data thereto.

Turning now to the flowchart of FIG. 2 of the appended drawings, theoperation of the power down arrangement of the IC 10 will be described.

In a first step 30, the system is reset. This is usually done when theIC is first powered. During the normal operation of the IC, it is in aso-called “normal” mode and the crystal oscillator supplies the clocksignal to the standard circuitry via the oscillator control circuit.

When the IC is reset, it causes the oscillator control circuit to obtainenvironmental data from the environmental sensors (step 32). This datais stored in a memory of the oscillator control circuit.

Step 34 is the measurement of the oscillation frequency of the low poweroscillator. It is to be noted that it has been found that themeasurement of the frequency of the low power oscillator isadvantageously made in a similar mode to that of the power down mode.Changes in the current draw on a battery (resulting in a supply voltagechange) create an error in the resulting measurement of the low poweroscillator frequency. Accordingly, a low power measurement mode is alsocreated where a minimal amount of circuitry is powered up to allow themeasurement of the low power oscillator to be made.

The low power oscillator frequency measurement step therefore includesthree sub-steps. First, the standard circuitry of the IC is shut down,then the frequency of the low power oscillator is measured using thecrystal oscillator as a reference and then the standard circuitry ispowered back up.

Eventually, the standard circuitry sends a power down signal to theoscillator control circuit (step 36). This signal instructs theoscillator control circuit that the standard circuitry wishes to enterthe power down mode for a predetermined period of time. The length ofthis period may be pre-programmed in the oscillator control circuit ormay be sent as a parameter in the power down signal.

When the oscillator control circuit receives the power down signal, itfirst acquires environment data from the environmental sensors andcompares this data to the last stored measurements (step 38).

In step 40, the oscillator control circuit verifies that theenvironmental data measured in step 38 is within predeterminedtolerances. It is to be noted that the tolerances vary from one set ofenvironmental data to another and are function of the type of low poweroscillator used.

If the measured environmental data is not within tolerances, step 42 isdone and the low power oscillator frequency is measured in a manneridentical to step 34.

In step 44, using the measured frequency of the low power oscillator instep 34 or 42, and knowing the predetermined power down time requestedby the standard circuitry, the oscillator control circuit sets a firstreal time counter so that it will know when to wake up the standardcircuitry. Since the oscillator control circuit knows how long the powerdown mode is to last and knows the frequency of oscillation of the lowpower oscillator, it can calculate the number of oscillations of the lowpower oscillator required and store this result in the first real timecounter.

Since the startup time of the crystal oscillator is significant and notprecise from one time to the next, a second real time counter is usedwith the low power oscillator so that the oscillator control circuitwill know when to wake up the crystal oscillator, a predetermined delaybefore the wake up of the standard circuitry. The difference of timebetween the first real time counter and the second real time counterbeing the length of the power up crystal oscillator mode as will bedescribed hereinbelow. As will easily be understood by one skilled inthe art, the length of time for the power up crystal oscillator mode canbe determined based on environment conditions provided by theenvironment sensors 22.

The IC then enters the power down mode in step 46. During the power downmode, the power to the standard circuitry and to the crystal oscillatoris shut off. Only the oscillator control circuit and the low poweroscillator draw power. More specifically, the oscillator control circuitincludes a state machine or microcontroller that is capable of carryingout the necessary algorithm for measuring and comparing environmentalsensor data and measuring low power oscillator frequency and calculatingthe power down time. The only sections of the oscillator controlcircuitry needed during power down are the real time counters and thecircuitry, which determines when to enter power up crystal oscillatormode and normal mode. Thus, only these sections of the oscillatorcontrol circuit and the low power oscillator draw power during powerdown.

Before the oscillator control circuit determines that it is time to wakeup the standard circuitry, the IC enters a power up crystal oscillatormode (step 48) where the crystal oscillator is energized. It stays inthis mode for a predetermined period of time (calculated and set in thesecond real time counter during step 44) to allow the output signal ofthe crystal oscillator to stabilize its frequency.

The normal mode is then resumed (step 50) by powering up the standardcircuitry that may accomplish its programmed functions (step 52) beforereturning to step 36 to repeat the cycle.

Optionally, the measured frequency of oscillation of the relaxationoscillator 16 may be used to measure the frequency of oscillation of thecrystal oscillator 14 during power up crystal mode to allow theadjustment of the bias currents of the crystal oscillator to optimizepower consumption versus the power up time.

Turning now to FIG. 3 of the appended drawings, a timing diagram showingthe basic operation of the power down system will be described.

As mentioned hereinabove and illustrated in FIG. 3, there are fourpossible modes: a power down mode 60, a power up crystal oscillator mode62, a measure oscillator frequency mode (not shown) and a normal mode64.

In the power down mode 60, only the low power oscillator and theoscillator control circuit are energized. Therefore, the crystaloscillator is shut down and the power consumption is minimal.

In the power up crystal oscillator mode 62, the low power oscillator,the crystal oscillator and the oscillator control circuit are energized.The total power consumption is therefore higher than the power downmode. As can be seen in the crystal oscillation diagram, the frequencyof the crystal oscillation becomes stable during this mode.

In the measure oscillator frequency mode, described hereinabove but notshown in FIG. 3, only the two oscillators and the oscillator controlcircuit are powered up. This mode is relatively short and used only whenthe frequency of the low power oscillator has to be measured.

Finally, in the normal mode 64 the standard circuitry is brought online.Therefore, the total power consumption is higher than during the othermodes. It is to be noted that the low power oscillator may be turned offduring the normal mode to reduce noise and to reduce the powerconsumption.

Although the present invention has been described hereinabove by way ofpreferred embodiments thereof, it can be modified, without departingfrom the spirit and nature of the subject invention as defined in theappended claims.

1. A power down system for an integrated circuit that enables a powerdown mode to be maintained for a predetermined time; said power downsystem comprising: an oscillator; a low power oscillator; at least oneenvironmental sensor; and an oscillator control circuit controlling bothsaid oscillator and said low power oscillator and receiving data fromsaid at least one environmental sensor; said oscillator control circuitincluding a real time counter; said oscillator control circuit being soconfigured that said oscillator is energized when said oscillatorcontrol circuit is in a normal mode; said oscillator control circuitbeing so configured that: when a reset signal is received: environmentaldata is acquired by said oscillator control circuit and stored in afirst memory; and said oscillator control circuit measures anoscillation frequency of said low power oscillator and stores thefrequency in a second memory; when a power down signal is received:environmental data is acquired by said oscillator control circuit andcompared with the stored environmental data; should the acquiredenvironmental data differ from the stored environmental data by at leasta predetermined tolerance, said oscillator control circuit measures anoscillation frequency of said low power oscillator and overwrites thedata contained in the second memory; said oscillator control circuituses the oscillation frequency of said low power oscillator stored inthe second memory to set said real time counter so as to maintain thepower down mode for the predetermined time, and said oscillator controlcircuit turns off said oscillator and uses said low power oscillator forthe duration of the power down.
 2. The power down system recited inclaim 1, wherein said oscillator is a crystal oscillator.
 3. The powerdown system recited in claim 1, wherein said low power oscillator isselected from the group consisting of relaxation oscillators, ringoscillators and RC oscillators.
 4. The power down system recited inclaim 1, wherein said low power oscillator includes a Schmitt triggeroscillator.
 5. The power down system recited in claim 1, wherein said atleast one environmental sensor is selected from the group consisting oftemperature sensors, IC process variation sensors and voltage sensors.6. The power down system recited in claim 1, wherein said oscillatorcontrol circuit further includes a second real time counter; andwherein, when a power down signal is received, said oscillator controlcircuit sets said second real time counter using the measuredoscillation frequency of the low power oscillator so that saidoscillator control circuit may turn on the oscillator before the powerdown is terminated to allow an oscillation frequency of said oscillatorto stabilize.
 7. A power down method for an integrated circuit thatenables a power down mode to be maintained for a predetermined duration;said integrated circuit including an oscillator, a low power oscillator,at least one environmental sensor, standard circuitry and an oscillatorcontrol circuit having a real time counter, memory and controlling boththe oscillator and the low power oscillator and connected to the atleast one environmental sensor; said power down method comprising:measuring initial environmental data; measuring an initial oscillationfrequency of the low power oscillator and storing it in memory;receiving a power down signal; measuring environmental data andcomparing with the initial environmental data; should the environmentaldata differ from the initial environmental data by at least apredetermined tolerance, measuring an oscillation frequency of the lowpower oscillator and overwrite the initial oscillation frequency inmemory; setting the real time counter as a function of the oscillationfrequency in memory so as to maintain the power down mode for thepredetermined duration; and turning off the oscillation of theoscillator and the standard circuitry for the duration of the power downmode, as determined by the real time counter.
 8. The power down methodrecited in claim 7, wherein said initial environmental data measuringact includes transferring environmental data from the at least oneenvironmental sensor to the oscillator control circuit.
 9. The powerdown method recited in claim 7, wherein said environmental datameasuring act includes transferring environmental data from the at leastone environmental sensor to the oscillator control circuit.
 10. Thepower down method recited in claim 7, wherein the predetermined durationof the power down mode is fixed.
 11. The power down method recited inclaim 7, wherein the predetermined duration of the power down mode issupplied within the power down signal.
 12. The power down method ofclaim 7, wherein said initial oscillation frequency measuring actincludes shutting down the standard circuitry of the integrated circuitwhile the measurement of the initial oscillation frequency is done. 13.The power down method of claim 7, wherein said oscillation frequencymeasuring act includes shutting down the standard circuitry of theintegrated circuit while the measurement of the oscillation frequency isdone.
 14. The power down method of claim 7, wherein the oscillatorcontrol circuit includes a second real time counter; said methodincluding: setting the second real time counter using the oscillationfrequency of the low power oscillator to a duration shorter that thepredetermined duration of the down time; when the shorter durationexpires, turning on the oscillator of the integrated circuit to allow anoscillation frequency thereof to stabilize; when the predetermined downtime duration expires, turning on the standard circuitry of theintegrated circuit.
 15. A power down system for an integrated circuitthat enables a power down mode to be maintained for a predeterminedtime; said integrated circuit including standard circuitry; said powerdown system comprising: an oscillator; a low power oscillator; and anoscillator control circuit controlling both said oscillator and said lowpower oscillator; said oscillator control circuit including a real timecounter; said oscillator control circuit being so configured that: saidoscillator is energized when said oscillator control circuit is in anormal mode; when a measurement of an oscillation frequency of said lowpower oscillator is required, the standard circuitry of the integratedcircuit is turned off; when a power down signal is received: a) saidoscillator control circuit uses the measured oscillation frequency ofsaid low power oscillator to set said real time counter so as tomaintain the power down mode for the predetermined time, and b) saidoscillator control circuit turns off said oscillator and uses said lowpower oscillator for the duration of the power down.
 16. The power downsystem recited in claim 15, wherein said oscillator is a crystaloscillator.
 17. The power down system recited in claim 15, wherein saidlow power oscillator is selected from the group consisting of relaxationoscillators, ring oscillators and RC oscillators.
 18. The power downsystem recited in claim 15, wherein said low power oscillator includes aSchmitt trigger oscillator.
 19. The power down system recited in claim15, wherein said oscillator control circuit is so configured that ameasurement of an oscillation frequency of said low power oscillator isrequired when the integrated circuit is reset.
 20. The power down systemrecited in claim 15, wherein said oscillator control circuit is soconfigured that a measurement of an oscillation frequency of said lowpower oscillator is required when environmental changes occur.
 21. Thepower down system recited in claim 15, wherein said oscillator controlcircuit further includes a second real time counter; and wherein, when apower down signal is received, said oscillator control circuit sets saidsecond real time counter using the measured oscillation frequency of thelow power oscillator so that said oscillator control circuit may turn onthe oscillator before the power down is terminated to allow anoscillation frequency of said oscillator to stabilize.
 22. A power downmethod for an integrated circuit that enables a power down mode to bemaintained for a predetermined duration; said integrated circuitincluding standard circuitry, an oscillator, a low power oscillator andan oscillator control circuit having a real time counter and controllingboth the oscillator and the low power oscillator; said power down methodcomprising: receiving a power down signal; when predetermined conditionsare met, shutting down the standard circuitry; measuring an oscillationfrequency of the low power oscillator; powering up the standardcircuitry; setting the real time counter as a function of the measuredoscillation frequency of the low power oscillator so as to maintain thepower down mode for the predetermined duration; and turning off theoscillation of the oscillator and shutting down the standard circuitryfor the duration of the power down mode, as determined by the real timecounter.
 23. The power down method recited in claim 22, whereinpredetermined conditions include a modification of environmentalconditions of the integrated circuit.
 24. The power down method recitedin claim 22, wherein predetermined conditions include a reception of areset signal.
 25. The power down method recited in claim 22, wherein theoscillator control circuit includes a second real time counter; saidmethod including: setting the second real time counter using theoscillation frequency of the low power oscillator to a duration shorterthat the predetermined duration of the down time; when the shorterduration expires, turning on the oscillator of the integrated circuit toallow an oscillation frequency thereof to stabilize; when thepredetermined down time duration expires, turning on the standardcircuitry of the integrated circuit.